Proceedings of the Iowa Academy of Science
Volume 69 Annual Issue Article 55
1962
Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quarry Near Denver, lowa
Fred H. Dorheim Iowa Geological Survey
Don L. Kock Iowa Geological Survey
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Recommended Citation Dorheim, Fred H. and Kock, Don L. (1962) "Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quarry Near Denver, lowa," Proceedings of the Iowa Academy of Science, 69(1), 341-350. Available at: https://scholarworks.uni.edu/pias/vol69/iss1/55
This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact [email protected]. Dorheim and Kock: Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quar
1962] PHOTOINTERPRETATION 341
Literature Cited Black, R. F. (in manuscript). Ice wedges and pennafrost of the Arctic Coastal Plain of Alaska. U. S. Geo!. Survey Prof. Paper. l Iop1
Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quarry Near Denver, lowa 1
FRED H. DoRHEIM2 AND DoN L. KocH2
Abstract. In the Loomis quarry, located in the NW1.4 sec. 29 T. 91N., R. 13W., Bremer County, Iowa, there is an un usual exposure of an unconformity between the Silurian and the Devonian sediments. Several Niagaran hummocks rise approximately 30 feet above the general Silurian level with Devonian sediments (Cedar Valley and Wapsipinicon) dcsposited against the Hanks of the Niagaran remnants. A basal Wapsipinicon breccia is exposed in this section. Chert from the Silurian forms an important part of the breccia.
INTRODUCTION A most remarkable exposure of an irregular Silurian surface with Devonian sediments deposired along the eroded flanks can be seen in the Charles Loomis quarry located in the NWl/4 sec. 29, T. 91N., R. l3W., Bremer County, Iowa. This quarry is gen erally known as the Denver Quarry. The location of the quarry and the routes leading to it from either Waverly or Denver are shown in figure 1. This study was made possible through the cooperation of Beu
1 Presented with the pennission of Dr. H. Garland Hershey, State Geologist, and Director of the Iowa Geological Survey. " Iowa Geologicfl\ Siµyey, Iowa City, Iowa. Published by UNI ScholarWorks, 1962 1 Proceedings of the Iowa Academy of Science, Vol. 69 [1962], No. 1, Art. 55
342 row A ACADEMY OF SCIENCE [Vol. 69
R 14 W R 13 W
z a; N ,_ c--·· I I I I I I
0 2 miles~--~-~ FIG. I - LOCATION MAP OF DENVER QUARRY and Sons Limestone Company, who opened and operated the quarry, and of the owner, Mr. Charles Loomis. The work was done as a part of Iowa Geological Survey official activities. As sociates at the Iowa Geological Survey have made helpful sug gestions. Dr. Brian F. Glenister, State University of Iowa, helped in the identification of fossils.
DISCUSSION General The locations of the various features to be discussed are shown in figure 2. Of special note is the Niagaran hummock about mid way on the west wall flanked on the north by sediments of the \Vapsipinicon Formation. Hummock, as used here, refers to elevations above the general Silurian top, either as conspicuous rises or as lesser undulations. Two Niagaran hummocks occur in the east face of the quarry with Devonian sediments deposited adjacent to each of them. A fourth hummock is exposed in the stripped area northeast of the quarry. Here an area of 0.1 acre of Niagaran rock can be seen surrounded by Devonian sedi ments. Stratigraphy and Lithology The following stratigraphic section was measured along the north wall of the quarry (fig. 2). The bed numbers used in this section are the same as those used on the graphic section (fig. 3). https://scholarworks.uni.edu/pias/vol69/iss1/55 2 Dorheim and Kock: Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quar
1962] SILURIAN-DEVONIAN CONFORMITY 343
Location at which basic jii;ipped stratigraphic section Nlagaran hummock expased Area was measured I r as Isolated mound; about .1 I { I 0.1 acre 1... __ ) Cedar Valley and Cedar Valley and Wapsipinicon over adjacent to Niagaran on north side of hummock Niagaran hummocks
Wapsipinicon on south side of hummock Saddle-Wapsipinicon over th inning Niagaran---'
Large Niagaran / hummock~
Scale (feel)
50 100 200
Fig. 2 - Map of Denver Quarry, showing location of Niagaran hummocks. Stratigraphic Section as Observed Along North Wall of Quarry Bed Description Thickness Pleistocene Loess and till ...... about 12' Devonian System Cedar Valley Formation. 1. Limestone, grayish-orange (IOYR7 /4) to very pale orange ( 10YR8/2), medium elastic, with numerous brachiopod frag ments and a few crinoidal stem sections. Badly weathered reticulate atrypoid brachiopods are locally abundant in lower part of bed. Dendrites of manganese oxides occur throughout. The heels are platy and badly slumped with geest distributed throughout ...... 10' max. 2. Breccia composed of fragments of grayish-orange ( 10YR7 I 4) calcareous dolomite, soft and friable, in a matrix of calcareous shale. Most of the fossils are very poorly preserved. Among those collected are Productella, Schizophoria meeki Fenton & Fenton, Atrypa devoniana Webster, and A. lineata Stain brook. Manganese oxides occur as scattered specks along with glauconite in the upper part of this bed and are abundant in the lower part . . 15' \Vapsipinicon Formation .3. Limestone, very pale yellowish-brown ( 10YR7 /2) to yellow ish-gray ( 5Y7 /2), lithographic to very fine crystalline, lamin- ated to brecciated with large solution cavities (fig. 8-A) and some flutings. The breccia is composed of small blocks of lithographic limestone in a limestone matrix. Bedding is irregular ...... lY Published by UNI ScholarWorks, 1962 3 Proceedings of the Iowa Academy of Science, Vol. 69 [1962], No. 1, Art. 55
.344 10\VA ACADEMY OF SCIENCE [Vol. 69
.. >a:: c <( ~ z s a:: :ii ~ a.. 0
LEGEND
l.imestcne
dolomitic
Dolomite aroillaceous argillaceous, and sandy cherty
Shale
-- brachiopods EB crlnolds e> corals ~z <( ~ "O Vertical Scale (feel) c a:: 0 ::::> ..)( ....I <( Cf)
Fig. 3- Stratigraphic section as exposed in north face of quarry and in sump along west face.
4. Limestone, as in bed .3 but more massive and with irregular solution cavities . 6-8' 5. Dolomite, pale yellowish-orange ( 10YR8/6), soft structure less, friable. Thirty percent of an insoluble residue sample consisted of clay and silt. A thin layer of dark-brown to orange to gray clay, silt, and sand caps the dolomite. \Vhen treated with acid about .30 percent of a sample from this layer digest ed. The remaining 70 percent was two-thirds clay and one- third quartz silt. Total thickness of bed 5 varies from . . . . .3J~-5' 6. Limestone,_ vari-colorecl, mostly yellowish-gray ( 5Y7 /2), very dolomitic, very argillaceous; with a few grains of rounded quartz sand and angular grains of dark gray chert sand. Less than 10 percent of the sample is composed of very sharp chips https://scholarworks.uni.edu/pias/vol69/iss1/55 4 Dorheim and Kock: Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quar
1962] SILURIAN-DEVONIAN CONFORMITY 345
of white ( N9) chert. Thc,e whit« chert l'hip; were derived ~1 from the Silurian . . . 1 .. ) 7. Limesotne hrecda (fi,g. 8-B), grayish-yellow (5Y8/4) to Vl'ff pale orange ( 10YH8/2 ), Vt•ry coarse, fragmental; with n fo~,. sane! grains, angular gray chert, and larg'' angular d1ips of ~1 white chert I ' ,') Silurian System Alexandrian Series 8. Dolomite, grayish-orange ( 10YR7 / 4) to yellowish-gray ( 5Y- 7.12), medium-fine, ha rel; with crystalline· calcite and chert nodules. The chert is medium-gray ( i\-6) to yellowish-gray ( .5Y7 /2) in the center of tlw nodule but wc,1tlwrs white on the surface. Silkifiecl Stromatopora and corals common 18'
Soutr. North
o' 200' 300' Fig. 4-Diogrom of west wall of quarry showing large Silurian hummock with Devonian beds deposited on and adjacent to the eroded surface
It is significant that neither the Niagaran nor the upper 8 feet of the Alexandrian is represented in this section. The Silurian rises most conspicuously above the quarry floor in the large hummock on the west side (figs. 4 and 6), nearly to the level of the top of bed 2 (fig. 3), the middle of the Cedar Valley, as exposed in the quarry. At this point there is approxi mately 2.5 feet of Niagaran underlain by about 8 feet of inter bedded chert and dolomitic limestone of the Alexandrian. The eroded Niagaran diminishes in thickness to the south and dis appears toward the southern encl of the quarry. The north side of this hummock falls vertically for approximately 17 feet, where the \Vapsipinicon lies unconformably on the thin remnant of Niagaran. Two Niagaran hummocks occur on the east side of the quarry (figs. 5 and 7) with a chert rubble deposited between them. This chert rubble is present on the north end of the northern hummock covered by a normal vVapsipinicon and Cedar Valley sequence. The southern hummock is flanked on the south with chert rubble and irregular or broken blocks of basal \Vapsipinicon breccia above it. Published by UNI ScholarWorks, 1962 5 Proceedings of the Iowa Academy of Science, Vol. 69 [1962], No. 1, Art. 55
346 IOWA ACADEMY OF SCIENCE [Vol. 69
North So;ith
40'
50' Alexandrian
0 100' 200' 300' 400' Fig. 5- Diagram of east wall of quarry showing two Silurian hummocks with Devonian beds deposited on and adjacent to the eroded surface. The chert rubble is post-Niagaran, pre-Wapsipinicon. At pres ent there is not enough evidence to support a closer age deter mination.
PALEONTOLOGY Silicified Favosites and Stromatopora occur abundantly in the Silurian strata represented in this quarry. Rare, but present in the Niagarian, are a few poorly preserved Pentamerus ablongus. The following fossils were collected from the Cedar Valley exposed in the north face (fig. 6-A) and at the north end of the eist face (fig. 5) : Atrypa devoniana Webster, A. lineata Strain brook, Schizophoria meeki Fenton & Fenton, Productella, Gypi dula, and small, high spired, turreted gastropods. The fossils collected from these beds are representative of the Solon mem ber of the Cedar Valley and, more particularly, of Stainbook's "independensis zone" of the Solon. Atrypa devoniatui Webster has little value as a zone fossil unless the variety can be determined. According to Stainbrook ( 1940) Schizophoria meeki Fenton & Fenton is an "independensis zone" fossil and Productella and Gypidula are characteristic of the lower Solon of the North Iowa section.
SPECIAL LITHOLOGIES There are several lithologic characteristics in the Wapsipinicon Formation that are worthy of mention. In parts of the Davenport member (fig. 3, beds 3 and 4), solution has created a gross cavernous porosity and some fluting. This is shown in figure 8-A. A basal W apsipinicon breccia is well developed in this quarry and is illustrated in figure 8-B. The large white, angular chert particles are derived from the cherts of the Silurian. Since the https://scholarworks.uni.edu/pias/vol69/iss1/55 6 Dorheim and Kock: Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quar
1962] SILURIAN-DEVONIAN CONFORMITY 347
A
8
Fig . 6 .- Denver quarry. A. North and west faces, looking north . 8 . West wall, looking south. The large Silurian hummock stands out from Devonian bed at right Niagaran, as observed in the quarry, is chert free these particles must have been incorporated from an Alexandrian chert residium in close proximity. The particles are too sharp to have been Publishedtransported by UNI ScholarWorks, far. 1962 7 Proceedings of the Iowa Academy of Science, Vol. 69 [1962], No. 1, Art. 55
348 IOWA ACADEMY OF SCIENCE [Vol. 69
A
8
Fig. 7.- East face . A . Northern Niagaran hummock with Devonian beds on north flank and chert rubble on south flank. B. Southern Niagaran hummock is flanked by chert rubble on north and south and also by Wapsipinicon on south
CONCLUSIONS The hummocky Silurian surface with its flanking Devonian sediments depicts an unconformable contact, illustrative of the pre-Wapsipinicon erosion and the post-Silurian sedimentary environment. This feature, though probably quite extensive, is not usually exposed, especially in glaciated areas. The "chert rubble" (fig. 5) appears to be an allogenic deposit derived from a weathered Alexandrian terrain and deposited on https://scholarworks.uni.edu/pias/vol69/iss1/55the Niagaran surface. This deposition may have occurred any 8 Dorheim and Kock: Unusual Exposure of Silurian-Devonian Unconformity in Loomis Quar
l962] SILURIAN-DEVONIAN CONFORMITY 349
A
B
Fig. 8 .-Close-up of portion of north face of quarry A. Extreme solution in Davenport limestone. B. Basal PublishedWapsipinicon by UNI ScholarWorks, breccia 1962. 9 Proceedings of the Iowa Academy of Science, Vol. 69 [1962], No. 1, Art. 55
350 IOWA ACADEMY OF SCIENCE [Vol. 69
time from post-Niagaran to pre-Wapsipinicon. The rubble lie~ on eroded Niagaran and below the Wapsipinicon breccia.
Literature Cited Stainbrook, M. A., 1942, Orthid Brachiopods of the Cedar Valley Limestone of Iowa. The American Midland Naturalist, vol. 23, no. 2, p. 492.
A Late Wisconsin Giant Beaver in Northern Iowa
w. D. FRANKFORTER1
Abstract. Few discoveries of giant beaver, Castoroides, re mains have been made in Iowa and these have usually been found under circumstances precluding determination of their geologic ages. The specimen reported herein from near Garner, Hancock County, Iowa, was recovered from a peat bed which lies within the border of a late Wisconsin, prob ably Cary, terminal moraine. According to radiocarbon dates recently announced for organic muck overlying till in a near by peat bog the minimum date for the till would have to be approximately 12,000 b.p., thus older than Two Creeks inter stadial. This find, places the giant beaver in Iowa during Two Creeks time or possibly slightly later. This is tlrn latest sur vival yet reported for this form in Iowa and compares favor ably with its terminal date reported in Ohio. Other giant beaver finds in Iowa are described and some general statements are made about the development of this animal.
INTRODUCTION Remains of the giant beaver, Castoroides, have been rarely found in Iowa. A recent discovery near Garner, Hancock County, is only the seventh specimen to come to the attention of the author. Four have been mentioned previously in scientific re ports and the remainder are discussed herein. The Garner specimen consists of a fragment of the upper left third molar, the lower left second molar, and a partial in cisor from the left ramus. These items were found bv Mr. Richard Brouwer in a peat deposit on his farm two miles. south and one mile \vest of Garner, in the Sl/z, NEl/4, sec. 11, T. 95N., R.24W. They were discovered in 1960 and were taken to Robert Brower of Garner, who was then a student at the State University of Iowa, Iowa City. He, in turn, submitted them to the author for identification and, subsequently, they were donated to San-
1 Sanford Musewn, 117 East Willow, Cherokee, Iowa. https://scholarworks.uni.edu/pias/vol69/iss1/55 10